1. Field of the Invention
This invention relates broadly to security systems and intrusion detector used therein. More particularly, this invention relates to fiber optic intrusion detectors.
2. State of the Art
Intrusion detectors are widely used in security systems to monitor the boundaries of a well-defined area in order to detect the presence, location and motion of people and vehicles. Exemplary applications for such intrusion detectors include the monitoring of the perimeters of national boundaries, military facilities, chemical plants, airports, rail stations and correctional facilities. One of the challenges for these detectors is the need to operate remotely in harsh environments with exposure to wide temperature ranges as well as rain, snow, and dirt.
Fiber optic sensors have been developed for intrusion detection. The fiber optic sensor has inherent advantages in that the fiber optic sensing element is passive (it does not carry electricity), which is particularly important for facilities with highly combustible materials. The fiber optic sensing element can also span over extended lengths (e.g., tens of kilometers). The fiber optic sensing element is immune to electromagnetic effects that might otherwise damage or interfere with its operation. And the fiber optic sensing element is readily available at competitive prices and in ruggedized cables capable of withstanding harsh environments.
Fiber optic intrusion detection systems are commercially-available from Future Fibre Technologies Pty Ltd of Mulgrave, Victoria, Australia and Fiber Sensys of Hillsboro, Oreg., USA. The Future Fibre Technologies system operates using a fiber optic loop including a forward path and a return path. The forward path includes two separate optical fibers. The return path includes a single optical fiber. The two optical fibers of the forward path form the arms of an interferometer. Continuous laser light is sent down the two arms of the interferometer. The light returned by the return path is analyzed. If there has been no external interference (motion, sound or vibrations) acting on the two arms of the interferometer, the return light will not change. If there is an external interference acting on the two arms of interferometer, the return light will change and an interference pattern generated. A controller detects this change and will interpret the effect as either an intrusion event or an ambient condition. The Fiber Sensys system injects coherent light into a multimode fiber. The mode of the light disperses along its length and mixes at the fiber's terminus, resulting in characteristic pattern of light and dark splotches called speckle. The laser speckle is stable as long as the fiber remains immobile, but flickers when the fiber is vibrated due to environmental effects (such as a person or vehicle passing nearby). Intrusion detection is accomplished by analyzing the speckle pattern over time. In either system, a break in the fiber optic sensor completely disables the intrusion detection system. Moreover, either system cannot detect and report the position of the fiber break.
An alternative approach is proposed in U.S. Pat. No. 5,194,847 to Taylor et al. In the Taylor system, light from a highly-coherent pulsed laser is launched into a sensing optical fiber. As the individual pulses propagate within the fiber in the forward direction, normal Rayleigh scattering causes a proportion of the light to be scattered uniformly, with a small fraction being recaptured by the fiber before it propagates in the reverse direction to the receiver. The coherent (narrow linewidth) nature of the launched pulses ensures that detectable optical interference can take place between the components of the scattered light. The system analyzes the phase changes and corresponding time delays of the backscatter signal in order to collect a spatial distribution of localized disturbances along the sensing fiber. In the static case, the spatial distribution is random but stable. In the dynamic case (which can be caused, for example, by a disturbance by an unauthorized intruder or vehicle), the localized pattern changes. Such changes can be used to indicate the occurrence of an intrusion and the approximate location of the intrusion along the sensing fiber. In this system, a break in the fiber would disable the capability for intrusion detection at points beyond the break. Such limitations hinder the deployment of such systems in critical security applications and provide opportunities for organized groups (terrorists, thieves and other undesirable third parties) to quickly disable these systems.
Thus, there remains a need in the art for fiber-optic based intrusion detection systems that can operate without interruption in the event that a break occurs in the sensing optical fiber of the system.